Electric motors, particularly ac induction motors, are employed in many industrial and manufacturing facilities. Among their many applications, ac induction motors are used to provide power to machinery in manufacturing facilities. Downtime caused by a failure of an electric motor reduces productivity and profitability. Electric motors, therefore, are important elements of industrial facilities and their health and condition must be closely observed to prevent motor failures that result in costly unscheduled downtime.
Evaluating the extent of stator wire insulation degradation has long been considered an effective way of determining the condition of an electric motor. As the motor ages, insulation breakdown occurs due to high motor temperature and other operating stresses. When insulation degradation reaches a critical point, the motor windings short circuit, thereby resulting in motor failure.
Attempts have been made in the art to provide electric motor monitoring systems capable of monitoring the condition of the motor. Many of these systems focus on determining the amount of insulation degradation as a way of predicting the remaining useful life of the motor. For example, in U.S. Pat. No. 5,019,760 to Chu et al., a system is disclosed for continuously determining the consumed life of electrical motor winding insulation. Insulation degradation is calculated as a function of insulation thermal properties, insulation temperature, and motor power rating. The temperature of the insulation is measured over a time interval, and the average winding temperature is used to calculate a consumed life of the insulation for that time interval. The consumed life values calculated for each time interval are then summed to determine a total consumed life of the motor.
In U.S. Pat. No. 5,189,350 to Mallett, there is described a monitoring system for an electric motor. A temperature sensor monitors the operating temperature of the motor. A memory is provided for storing the "absolute maximum operating temperature" and a "predetermined maximum permitted operating temperature" of the motor. An indicator is used for indicating the sensed motor temperature as lying within either a safe, hazardous, or dangerous range as determined by comparison of the sensed motor temperature to the stored maximums. A recorder stores the number of times the motor has operated beyond the predetermined maximum permitted operating temperature and the number of times the motor has operated beyond the absolute maximum operating temperature.
In U.S. Pat. No. 4,525,763 to Hardy et al., a system for predicting the remaining useful life of an electric motor is described. Hardy utilizes motor temperature and past history to determine the amount of insulation degradation, which forms the basis of the remaining useful life prediction. The predicted remaining useful life is displayed and may be used to issue a warning or to trip circuit breakers supplying power to the motor when projected life is shorter than predicted life.
While high motor temperature, and resultant insulation degradation can contribute to electric motor failures, it is not the only factor. There are many complex and interrelated operating characteristics of electric motors that affect the health and longevity of the motor. Factors such as motor speed, loading, vibration, and the number of motor starts/stops also affect motor life. However, all known electric motor monitors today are incapable of monitoring these operating characteristics in an efficient, productive manner.
Known prior art devices do not adequately address multiple contributing factors to motor failure, maintenance issues for an electric motor monitoring system itself, enhancing the useability of a monitoring system, or providing a monitoring system that can be applied in a cost effective manner.
What is needed, therefore, is an electric motor monitor capable of sensing and analyzing various stresses experienced by the motor during the life of the motor, including temperature, and storing these stress data as the operation history of the motor. The monitor should be configured to allow stored data to be easily downloaded for archival or further analysis. For purposes of maintainability and affordability, the monitor should be relatively small and self-contained with its own internal power source, and capable of being mounted directly to the motor. Finally, with the monitor mounted directly to the motor, it should be rugged and capable of withstanding the rigors of a harsh industrial environment.